OVA 1 & 2 is putting the main story of episodes together, OVA 3 & 4 is the continue of the rest of the story.
Alucard, himself is a intelligent being, as he lived over thousand years and watching human doing foolish stuff. He teaches his 'way of living' or 'the way human should behave' in some episodes. He view most human as dog or monster as they 'doesn't' have the right to be human.
This show is bloody, weaponary and can't be explained using these words above. You will understand it after you watched it.